1. Field of the Invention
The present invention relates to an optical fiber device that receives an input optical pulse having a predetermined center wavelength and outputs an optical pulse having a wavelength bandwidth broader than that of the input optical pulse.
2. Description of the Related Art
As transmission capacity and signal speed increase in optical communication, an all-optical processing technology gets an attention as a key technology for the future, in which an optical signal is transmitted as it is without being converted into an electric signal by a transceiver or a repeater. As a device for performing the all-optical processing technology, an optical fiber device using a nonlinear effect in an optical fiber and a device using semiconductor are being studied.
A supercontinuum (SC) light source generates a SC light, i.e., an optical pulse with a broadband spectrum of tens to hundreds nanometers. It is also expected to be used as a light source for broadband optical communication and a light source for sensing, for example, an optical coherent tomography (see, for example, Hidehiko Takara, et al., “Multi-Carrier Generation from a Single Supercontinuum Source”, Laser Engineering, Vol. 30, No. 1, p. 33, January 2002).
The SC light source includes an optical pulse source, an optical amplifier, and an optical fiber. The optical pulse source generates an optical pulse, which is amplified by the optical amplifier and enters into an input facet of the optical fiber. The optical pulse causes the nonlinear effect in the optical fiber, which broadens a spectral bandwidth of the optical pulse, resulting in an emission of the SC light from an output facet of the optical fiber. A plurality of nonlinear effects is considered to be caused in the optical fiber including self-phase modulation, cross-phase modulation, four-wave mixing, and Raman scattering.
Optical properties of the SC light source are greatly affected by a wavelength dispersion of the optical fiber. There are two types of the SC light source depending on conditions for determining the wavelength dispersion of the optical fiber. One type uses a dispersion-decreasing optical fiber in which an optical pulse input to the input facet of the optical fiber changes continuously from anomalous dispersion to normal dispersion toward the output facet, as disclosed in K. Mori, et al., “Flatly Broadened Supercontinuum Spectrum Generated in a Dispersion Decreasing Fiber with Convex Dispersion Profile”, Electron. Lett., Vol. 33, No. 21, pp. 1806-1808, 1997. The other type uses an optical fiber that has uniform normal dispersion through its whole length, as disclosed in Y. Takushima, et al., “Generation of over 140-nm wide Super-Continuum from a Normal Dispersion Fiber by Using a Mode-Locked Semiconductor Laser Source”, IEEE Photon. Technol. Lett., Vol. 10, No. 11, pp. 1560-1562, 1998. The optical fiber with the normal dispersion is advantageous because of its high spectral flatness and high signal-to-noise ratio (SNR). Furthermore, the spectrum of the SC light generated in the normal dispersion fiber broadens from the center wavelength of the input optical pulse to longer and shorter wavelength areas. As the absolute value of the wavelength dispersion becomes small, the spectrum broadens more.
However, a general optical fiber has a positive wavelength dispersion slope, i.e., the wavelength dispersion increases as the wavelength increases. As a result, in an area of wavelength longer than a zero-dispersion wavelength, an anomalous dispersion is observed, i.e., the wavelength dispersion is positive. In the anomalous dispersion area, modulation instability (MI) occurs to the spectrum. Therefore, if the spectrum of the light broadens into the anomalous dispersion area, noises and ripples increase and the spectral flatness degrades. On the other hand, even when the spectrum broadens in the normal dispersion area, if the absolute value of the wavelength dispersion is too small, the spectral flatness degrades.
There is a method of increasing the bandwidth of the light in an optical fiber by using a long optical fiber and an intense optical pulse, whereby increasing the nonlinear effect. However, when the stage length of the optical fiber and the intensity of the optical pulse are increased, the stimulated Brillouin scattering (SBS) occurs notably. The SBS is a phenomenon in which a part of input light is scattered backward as a Brillouin-shifted light without being transmitted through the optical fiber. The SBS occurs when an intense light having intensity higher than a predetermined threshold is input to the optical fiber. Due to the SBS, even if the intense light is input, the nonlinear effect is not increased as desired in the optical fiber. Therefore, the intensity of the optical pulse input to the optical fiber is limited and the bandwidth of the SC light can hardly be broadened.